For example, in a process for fabricating semiconductor devices, various plasma processes, such as etching, sputtering and CVD (chemical vapor deposition), are used for processing semiconductor wafers which are substrates to be processed.
As plasma processing systems for carrying out such plasma processes, various systems are used. Among these systems, capacitive coupled parallel plate plasma processing systems are mainly used.
The capacitive coupled parallel plate plasma processing system has a pair of parallel plate electrodes (top and bottom electrodes) in a chamber. This system is designed to feed a process gas into the chamber and to apply a high frequency power to at least one of the electrodes to form a high frequency field between the electrodes to form plasma of the process gas by the high frequency field to plasma-process semiconductor wafers.
If a film, e.g., an oxide film, on a semiconductor wafer is etched by means of such a capacitive coupled parallel plate plasma processing system, the optimum radical control can be carried out by causing the pressure in the chamber to be a medium pressure to form a medium density plasma. Thus, it is possible to obtain an appropriate plasma state, so that it is possible to realize stable and repeatable etching with a high etching selectivity.
However, in recent years, the scale down of the design rule for ULSIs is increasingly advancing, and a higher aspect ratio of a hole shape is required, so that conventional conditions are not always sufficient.
Therefore, it has been attempted to raise the frequency of the high frequency power to be applied, to about 60 MHz to form a high density plasma to an appropriate plasma under lower pressure conditions to cope with the scale down. However, it is difficult to produce a high density plasma at a degree of vacuum of 10 mTorr or less by the frequency of about 60 MHz. For that reason, it has been studied that the frequency of the high frequency power to be applied is further raised to 70 MHz or higher.
By the way, in the capacitive coupled parallel plate plasma processing system for thus applying a high frequency power to form plasma, a matching unit for matching impedance of plasma, which is the load of a high frequency power, to a transmission path impedance is provided between a high frequency power supply and a top electrode. A conventional matching unit has a structure shown in, e.g., FIG. 16. That is, a matching unit 101 is provided between a high frequency power supply 100 and a top electrode 102, and has a grounded rectangular parallelopiped box 101a, in which a coil 111 and a variable capacitor 114 are provided in series upstream of a feeding rod 103 for feeding power from the high frequency power supply 100 to the top electrode 102. Moreover, a grounded fixed capacitor 110 is provided upstream of the coil 111, and a variable capacitor 112 and a fixed capacitor 113 which are grounded are provided downstream of the coil 111. These parts are connected by copper plates or wires. By changing the values of the variable capacitors 112 and 114, the matching range is changed. Variable coils may be substituted for the variable capacitors.
However, if a matching unit with such a structure is used in a frequency band exceeding 70 MHz, the influence of the inductive reactance components of the copper plate for connecting the parts and the feeding rod for connecting the matching unit to the electrode increases. On the other hand, since capacitive reactance is in inverse proportion to frequency, the capacity of capacitors for resonance and matching is very small, so that it is difficult to utilize commercially-available variable capacitors. This tendency becomes remarkable when the frequency of the high frequency power is 100 MHz or higher.
In order to avoid such inconvenience, it is considered that a matching unit of a stub system, which is usually used for matching in the range of from the second half of the VHF band to the UHF band (300 MHz to 3 GHz), is used for a plasma processing system in a frequency band exceeding 100 MHz. As shown in FIG. 17, a matching unit 121 of a stub system is designed to adjust impedance by moving short-circuiting elements 133 on two or more adjusting lines 132 having coaxial cable structures, which are connected to and arranged perpendicularly to a feeding line 131 having a coaxial structure for connecting a high frequency power supply 120 to a top electrode 122.
However, in such a matching unit 121 of the stub system, the stroke of the short-circuiting elements 133 must be ensured to be ¼ wavelength or more. In the case of 150 MHz or less, the length of the adjusting lines 132 must be 500 mm or more, so that the matching unit itself is very large. In addition, the traveling time for the short-circuiting element 133 increases, so that the time required to match after the turning-on of the high frequency power increases. Moreover, considering that the short-circuiting element 133 is driven by a motor, it is not possible to avoid the complicated structure, such as conversion of rotational motion to linear motion.
On the other hand, the conventional matching unit is connected to the electrode by the feeding rod having a length of tens to about 100 mm, and the parts in the matching unit are connected by the copper plates or the like. If the number of mechanical connecting portions is thus large, the number of places in which electric characteristics are discontinuous increases. Therefore, standing waves are also discontinuous, so that plasma is ununiform, and loss due to R components increases. In addition, since the inductive reactance of the feeding rod becomes very large by the rising of frequency, the voltage at the outlet of the matching unit becomes very high, so that it is required to increase the size of insulating materials and spaces to reinforce insulation.